Solar light has a spectrum over a wide energy range, as illustrated in FIG. 1. Solar cells made from a semiconductor generate electricity upon absorption of light having an energy that is equal to or greater than the band gap of the semiconductor. For example, crystalline silicon absorbs light having an energy of 1.12 eV or more, to generate a hole and an electron in the crystalline silicon. Under irradiation with constant light, the energy difference between the hole and the electron is determined by the band gap of the semiconductor, and the energy difference is 1.12 eV regardless of the wavelength of the incident light.
The open-circuit voltage Voc of the solar cell illustrated in FIG. 2 is determined by the hole-electron polarization efficiency in the semiconductor. Voc does not exceed the band gap (the value expressed in eV) of the semiconductor, but is a value close to the band gap.
In a case in which the semiconductor is irradiated with light having a short wavelength having an energy greater than the band gap, the energy of the incident light is partially lost in the semiconductor, and Voc is a small value that is close to the band gap. Therefore, the energy of light with which generation of electricity proceeds most efficiently is a value that is slightly greater than the band gap of the semiconductor solar cell. In other words, it is desirable, in terms of realizing high efficiency, to irradiate the semiconductor solar cell with light having a slightly shorter wavelength than the wavelength corresponding to the band gap.
In a case in which a solar cell having a small band gap has absorbed light having a short wavelength, the power generation efficiency is very low, for the following reasons. Specifically, even when the solar cell is irradiated with light having a short wavelength with a large photon energy, Voc is determined by the band gap of the solar cell, and does not increase. Further, since the photon energy is large, the number of photons decreases; as a result, Isc is small relative to a unit light irradiation intensity, and the power generation efficiency decreases.
Multijunction solar cells, in which plural semiconductors having mutually different band gaps and mutually different spectral absorption sensitivities are superposed one on another, have been developed with a view to obtaining a large electromotive force and a large electric power. For example, multijunction (laminated) solar cells in which p-n junctions of AlInP, InGaAs and Ge are formed by lamination using the epitaxial crystal formation technique as illustrated in FIG. 3 have been proposed (see Non-Patent Document 1: Japanese Journal of Applied Physics, Vol. 43, No. 3 (2004), pp. 882-889 “Evaluation of InGaP/InGaAs/Ge Triple-Junction Solar Cell under Concentrated Light by Simulation Program with Integrated Circuit Emphasis” authored by K. NISHIOKA, T. TAKAMOTO1, T. AGUI1, M. KANEIWA1, Y. URAOKA and T. FUYUKI). Such multijunction solar cells are capable of generating power by absorbing light in a wide region ranging from visible light to infrared regions.
Further, as illustrated in FIG. 4, a technique whereby cells having mutually different band gaps and prepared as single cells in advance are stacked one on the other and adhered to each other using a transparent electroconductive adhesive (see Non-Patent Document 2: J. Takenezawa, M. Hasumi, T. Sameshima, T. Koida, T. Kanko, M. Karasawa and M. Kondo, Extended Abt. of the 2010 Int. Conf on Sol. State Dev. and Mat. (Tokyo, 2010) 1-8-4).
Moreover, a solar cell module configured such that plural solar cell panels each having solar cells having mutually different spectral sensitivities disposed on both sides of a thin-plate-shaped substrate are arranged in parallel to each other with a predetermined spacing therebetween and such that light reflected by the surface of the first solar cell constituting one solar cell panel falls on the second solar cell of another solar cell panel (see Patent Document 1: Japanese Patent Application Laid-open (JP-A) No. 2007-287997).
Further, a multiconnection solar power generation device 100 utilizing the absorption characteristics of the solar cell itself has been proposed (see Patent Document 2: JP-A No. 2012-204673) which is equipped with, as illustrated in FIG. 5, plural solar cells 1, 2 and 3 having mutually different absorption wavelength ranges and electrically connected in series by wiring 4, and reflection mirrors 6 and 7 reflecting light coming from the exterior so as to irradiate each of the solar cells with light in the absorption wavelength range thereof to generate electric power.
A blind equipped with a solar cell in which solar cells having mutually different wavelength sensitivities are respectively provided on the front and rear sides of a light shielding plate, and in which the solar cell on the front side receives direct solar light whereas the solar cell on the rear side receives reflected light from the solar cell on the front side, and utilization of the electric power generated by this blind equipped with the solar cell for night illuminations through storage in a storage battery, have been proposed (see Patent Document 3: JP-A No. H2-308086).